NVE36船板钢连续冷却转变行为及动力学回归模型

采用热膨胀仪对船板钢NVE36进行了连续冷却转变曲线(CCT曲线)的测定,并用显微镜观察其室温组织,用维氏硬度仪测定了组织硬度.利用Matlab软件平台对实验数据进行处理,建立了相变点温度-冷却速率关系模型及动力学回归模型,回归计算得到该钢种的最优模型系数.最后对比了NVE36钢在连续冷却过程中实验和回归模拟的动力学行为.结果表明计算值与实验值吻合很好,证明所建立模型的合理性及数据处理方法的可行性.     

20CrMnTiH连续冷却相变预测模型

采用DIL805型淬火变形膨胀仪测定了三种不同成分20CrMnTiH实验钢在不同冷却速度下的热膨胀曲线,对室温显微组织进行观察,并绘制连续冷却转变（CCT）曲线。实验结果表明：成分波动主要影响20CrMnTiH钢冷却转变过程中贝氏体与马氏体相变冷却区间,对临界相变温度影响较小。采用K-M方程拟合了三种实验钢的马氏体相变动力学参数。结合优化的Li经验模型及临界转变温度的回归关系式,建立了20CrMnTiH钢在连续冷却过程中的铁素体、珠光体与贝氏体的相变预测模型,成功预测了成分波动对实验钢CCT曲线的影响。进而,采用有限元分析方法建立了20CrMnTiH钢端淬仿真模型,较好地预测成分波动对实验钢淬透性的影响,此方法可为齿轮钢的成分优化设计与合理选材提供参考。     

一种含Nb、Ti微合金钢的连续冷却相变行为

利用MMS -200热模拟试验机对试验钢进行不同变形条件下的连续冷却转变试验.针对不同的冷却速度,采用热膨胀法结合金相法测定试验钢在连续冷却转变中的相变温度,利用光学显微镜及扫描电镜观察试样的显微组织,并测定珠光体的含量,进而绘制出试验钢在变形和未变形条件下的连续冷却转变(CCT)曲线,从而得出不同变形工艺对微合金钢连续冷却转变行为和显微组织的影响规律.     

低合金高强钢形变奥氏体相变行为及其模型

 为了研究低合金高强钢的形变奥氏体连续冷却转变行为及组织变化规律，利用Gleeble-3800热力模拟试验机对试验钢进行了多道次轧制工艺模拟试验，并根据试验所得相变温度和各冷却速度下的室温组织相对量，回归出相变模型。结果表明，试验钢在变形后的连续冷却过程中发生铁素体、珠光体、贝氏体和马氏体转变，基于对各转变的温度和组织相对量得到的相变模型分析，证实了模型的可靠性。     

Nb、Mo元素对X70级管线钢组织和硬度的影响

 在Formastor-Ⅱ热膨胀相变仪上,测定了3种不同化学成分的试验钢在不同冷却速度下连续冷却时的热膨胀曲线,采用热膨胀法并结合金相-硬度法绘制了试验钢连续冷却转变曲线（CCT曲线）,研究了冷却速度和化学成分对组织及硬度的影响。结果表明：添加Mo元素和高铌含量的试验钢均能在较低的冷却速度和较大的冷速范围内获得针状铁素体组织,并且试验钢的硬度随冷却速度的增大而增加。     

17MnNiVNbR钢的连续冷却转变行为研究

利用Formastor-FII型膨胀仪和Gleeble-3800热模拟试验机,结合显微组织观察和硬度测试,研究了压力容器用钢17MnNiVNbR的静态和动态连续冷却转变行为,并分析了热变形对相变行为的影响。实验结果表明:冷却速率较低时,17MnNiVNbR钢的相变组织为先共析铁素体和珠光体;随着冷却速率的增加,依次出现贝氏体和马氏体。热变形能提高铁素体、珠光体和贝氏体的相变温度,并使连续冷却转变曲线向左上方移动。     

Q500q钢的连续冷却转变行为及生产试制

利用热模拟实验机Gleeble-2000,对Q500q钢连续冷却转变行为以及在650～300℃温度区间的相变行为进行了研究及生产试制.结果表明:当冷速为1～4℃/s时,试验钢的微观组织由铁素体和珠光体组成.当冷速增至4～16℃/s时,发生贝氏体相变;随着冷却速度的增加,贝氏体组织更为细化且体积分数增加.当冷却速度大于4℃/s后,试验钢在650～300℃冷却速度减半时,贝氏体相变的终了温度升高,贝氏体相变区间缩小,与连续冷却转变相比组织差距不大.采用两种不同的冷却方式生产试制后,两组试验钢的力学性能和金相显微组织一致,说明650℃以下可以采用缓冷坑堆冷的方式来提高钢板的探伤合格率.     

稀土对低碳高铌钢相变行为及组织的影响

利用L78 RITA热膨胀相变仪研究了不同冷速下稀土对低碳高铌钢相变行为的影响,并结合金相法绘制了加稀土和未加稀土两种成分试验钢的连续冷却相变曲线,分析了稀土对连续冷却组织的影响规律。试验结果表明,稀土加入低碳高铌钢后使连续冷却相变曲线向右移动,在相同冷速下显著降低了奥氏体向铁素体转变的开始温度,并且随着冷速的增加使降低程度增大。冷速从0.1℃/s增加到15℃/s时,加稀土的低碳高铌相变开始温度点下降了90℃,而未加稀土的下降了52℃;另一方面,稀土对低碳高铌钢在0.1℃/s到1℃/s冷速下的组织有较大影响,促进了针状铁素体的转变,并能在冷速增至5℃/s后使其更易获得板条状贝氏体组织。     

冷作硬化非调质钢的连续冷却相变规律

钢的连续冷却相变曲线(CCT)是组织调控的基本依据,为了优化紧固件用冷作硬化非调钢热轧态的组织和力学性能,采用DIL805A相变仪测定了试验钢在0.1～50℃/s不同冷却速率下的热膨胀曲线,结合金相硬度法确定相变类型,并绘制了试验钢的CCT曲线.结果 表明,试验钢马氏体转变点(Ms)为280℃,在不同冷速范围内均有铁素...     

冷却速率对T91钢相变过程及组织的影响

利用DIL805A/D高精度差分膨胀仪,通过线膨胀行为测量获得相关动力学信息,结合冷却后的组织特征,研究了T91钢不同冷却速度(2～6000℃/min)下过冷奥氏体的相变过程和产物,确定了该钢组织转变的临界冷却速度以及淬火速率对马氏体转变点及组织的影响,绘制了连续冷却转变曲线。研究表明:T91钢的连续冷却过程中只存在铁素体和马氏体转变区,10℃/min为马氏体转变的临界冷却速度。不同淬火速率对T91钢马氏体开始转变温度有较大的影响,它不同于随冷速增加而相变点升高的经典理论。淬火速率通过碳原子气团、内应力的形成来影响过冷奥氏体状态,从而影响相变点;随淬火速度的增加,过冷奥氏体转变后的组织呈细化趋势。     

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

 The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bainitic steel during continuous cooling. Continuous cooling transformation (CCT) curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln［1/(1-f)］ and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth patterns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agreement with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.     

Differential scanning calorimetry study and computer modeling of <Emphasis Type="Italic">β</Emphasis> ⇒ <Emphasis Type="Italic">α</Emphasis> phase transformation in a Ti-6Al-4V alloy

The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the β ⇒ α transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling-transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson-Mehl-Avrami (JMA) theory and by applying the “concept of additivity.” The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the β ⇒ α transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.     

Comparison of Austenite Decomposition Models During Finite Element Simulation of Water Quenching and Air Cooling of AISI 4140 Steel

An indigenous, non-linear, and coupled finite element (FE) program has been developed to predict the temperature field and phase evolution during heat treatment of steels. The diffusional transformations during continuous cooling of steels were modeled using Johnson–Mehl–Avrami–Komogorov equation, and the non-diffusion transformation was modeled using Koistinen–Marburger equation. Cylindrical quench probes made of AISI 4140 steel of 20-mm diameter and 50-mm long were heated to 1123 K (850 °C), quenched in water, and cooled in air. The temperature history during continuous cooling was recorded at the selected interior locations of the quench probes. The probes were then sectioned at the mid plane and resultant microstructures were observed. The process of water quenching and air cooling of AISI 4140 steel probes was simulated with the heat flux boundary condition in the FE program. The heat flux for air cooling process was calculated through the inverse heat conduction method using the cooling curve measured during air cooling of a stainless steel 304L probe as an input. The heat flux for the water quenching process was calculated from a surface heat flux model proposed for quenching simulations. The isothermal transformation start and finish times of different phases were taken from the published TTT data and were also calculated using Kirkaldy model and Li model and used in the FE program. The simulated cooling curves and phases using the published TTT data had a good agreement with the experimentally measured values. The computation results revealed that the use of published TTT data was more reliable in predicting the phase transformation during heat treatment of low alloy steels than the use of the Kirkaldy or Li model.     

Decomposition kinetics of supercooled austenite in porous powder steel

To determine the best heat treatment of steel, thermokinetic diagrams of austenite decomposition are required. Experimental determination of those diagrams is extremely difficult. In the present work, a model of the phase transitions is proposed; the model takes account of the structure of powder steel. A method is developed for computer experiments with steel cooling. The proposed model permits the determination of the kinetic parameters of austenite decomposition on the basis of isothermal diagrams for two values of the steel porosity. By that means, the transformation kinetics in this steel may be predicted for other values of the porosity, both in isothermal conditions and in continuous cooling. The model is verified for У8 and ПК80 steel with different porosity, by means of published experimental data.     

82A钢高速线材控制冷却工艺的优化

利用Gleeble 1500热模拟实验机测定了82A高碳钢(%:0.81C、0.50Mn、0.21Si、0.007P、0.006S)Φ5.5 mm线材在0.8～40℃/s冷却速度下的连续冷却转变(CCT)曲线,经与现场实测冷却曲线估算的实际相变温度拟合修正,得出符合生产实际的动态CCT曲线。结合高速线材控制冷却过程的基本模型,得出82A钢线材适宜的吐丝温度为910～930℃;相变过程最佳冷却速度为9～12℃/s。     

基于相变产物构建DP1180相变动力学方程研究

摘要：DP1180钢相变动力学方程的构建,有利于其力学性能的精准调控。利用Gleeble 3500对DP1180钢进行相变点测定,结合切线法、金相 硬度法研究了DP1180钢在冷却过程中的显微组织演变规律,绘制连续冷却转变曲线(CCT),并基于相变产物对相变动力学方程（JMAK方程）进行了修正。结果表明：冷速为0.5~1℃/s时,组织为铁素体(F)和贝氏体(B);冷速为2℃/s时,有马氏体(M)出现;冷速为10℃/s时,组织为贝氏体(B)和马氏体(M);冷速大于20℃/s时,组织以马氏体(M)为主;显微硬度随冷速的增加而升高;基于不同相变产物对n值的影响规律,对传统JMAK方程进行修正,构建了基于相变产物的相变动力学方程,预测精度得到提升。     

核岛用P280GH钢的冷却相变过程及数值模拟

 为了研究核岛用P280GH钢管热轧成形后的冷却过程组织演变,采用物理模拟方法建立了CCT和TTT相变动力学曲线。结果表明,低冷速会形成粗大铁素体和珠光体,增大冷速可细化低温相组织,当冷速超过10 ℃/s时即开始出现贝氏体组织。运用叠加原理,基于有限元法,建立了试验钢冷却过程数值模型,分析了[?]219.1 mm×18.26 mm规格P280GH钢管在空冷和水冷两种冷却过程沿壁厚方向温度场、组织演变规律。结果显示,空冷条件相变组织为均匀的珠光体、无贝氏体和残余奥氏体;水冷条件获得相变组织为马氏体组织,管内外表面体积分数相差3.6%,计算结果与实际热处理工况基本一致。研究结果可为热轧P280GH钢管生产的控冷工艺提供指导。     

奥氏体形变对50CrV4钢相变行为的影响

利用Thermecmastor-Z型热模拟试验机,结合金相显微镜(OM)、扫描电镜(SEM)、维氏硬度计等,系统研究了奥氏体区变形对50CrV4钢连续冷却相变和等温相变规律的影响。建立了试验钢动态CCT曲线。研究结果表明,奥氏体变形能促进连续冷却转变过程中铁素体-珠光体、贝氏体转变,但亦可提高奥氏体的机械稳定性,进而抑制马氏体转变,Ms点由331.6℃(奥氏体未变形)降低至291℃(950℃下变形50%+890℃下变形50%,变形速率均为5s-1,变形后冷速为20℃/s)。当轧后冷速小于0.5℃/s时,试验钢中可获得铁素体+珠光体组织。此外,在研究不同变形量对试验钢等温相变规律影响时发现,650℃等温时,试验钢中发生铁素体-珠光体相变。随着变形量的增加(由30%增加至50%),其等温相变动力学加快(相变完成时间由197.6s减小至136.5s),铁素体体晶粒尺寸、珠光体片层间距减小,硬度增加。     

低合金钢焊接粗晶区连续冷却铁素体相变规律

 利用焊接粗晶区连续冷却淬火方法,对比分析了钛处理钢和普通C Mn钢焊接粗晶区连续冷却不同阶段的相变组织,研究了铁素体相变规律。结果表明,C Mn钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体;钛处理钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体+晶内铁素体组织。在钛处理钢中,晶界铁素体、魏氏组织铁素体和晶内铁素体的相变开始温度相同,但各自长大的动力学条件不同。当晶内铁素体和魏氏组织铁素体竞争发生相变时,晶内铁素体在晶内弥散分布氧化物夹杂上的非均质形核抑制了魏氏组织铁素体向晶内的长大。     

含钼低碳微合金化钢连续冷却过程相变动力学研究

在热模拟试验机上采用膨胀法并结合金相组织观察，测定了含钼和不含钼的试验钢贝氏体转变CCT曲线。研究了超低碳微合金化钢中钼元素对形变奥氏体连续冷却转变过程相变动力学的影响，结果表明，钢中加入ω（Mo）0．4％后，明显降低贝氏体的析出温度，并得到更细小的贝氏体组织；同时在相同的冷速条件下形成的M（马氏体）岛状组织也更加细小弥散；形变奥氏体在连续冷却过程中即使在冷速较低情况下也能获得细小的贝氏体组织。